Allow to provide override values to `defaultValue()`

It would be useful to directly provide overrides to `defaultValue`, e.g.

```ts
const validator = ValidatorContainer.defaultValue({activationEligibilityEpoch: 10, ...});
```
and missing values use defaults. This makes generating testing data a bit easier and look cleaner.

Start by locating ValidatorContainer.defaultValue and the tests covering default-value generation. Verify that callers can provide partial override values, unspecified values still receive defaults, and the testing-data use case is covered by the tests.

Written by the indexing model from the issue text.